1. Field of the Invention
The present invention relates to position transducers and rotary motion encoders (resolvers) and more particularly to an inductive-type incremental angular position transducer having an improved stator and rotor winding geometry, compact packaging and/or a regular encoder output pulse train and a once-around index pulse for triggering other events in a machine.
2. Description of Related Art
A motion encoder is a device which produces an electronic signal whose frequency is proportional to the angular velocity of a member being measured (e.g., a shaft). Conventional encoders employ, for example, a very accurate optical disc. The disc can include a series of slots along its circumference or alternating transparent and opaque segments along its circumference which, when conveyed past a light beam, break the light beam and thereby create a pulse as the optical disc rotates. The frequency of the pulse varies as the speed of rotation of the disc varies. Alternatively, a single slot can be provided on the disc so that each output of the light pulse indicates 360.degree. of rotation of the disc. However, optical discs are expensive to manufacture accurately. Additionally, the alignment specifications required to achieve desired accuracy increases costs significantly and thus prohibits application in many cases.
Inductive-type rotary motion encoders employ an induction principle to create pulses as a rotor is rotated. In contrast to optical encoders, which employ a single photodetector, inductive type encoders average the angular position along the perimeter of the rotary disc. This decreases the sensitivity to alignment parameters by an order of magnitude over optical encoders. Thus, a principle advantage of inductive type rotary encoders is their tolerance to mechanical alignment. The influence of miscentering and tilt are greatly reduced because the rotor sums the contributions from individual stator coils located around the perimeter thereof.
Since optical encoders have only one detector, no averaging (or summing) around the rotor perimeter can be performed. This is the primary pitfall of optical encoders. While the accuracy specification of an optical encoder may be 0.25 minutes of arc, even with extreme care, this accuracy can be achieved in practice only with great care in alignment. The best expected accuracy achievable with optical encoders is about 1-2 minutes of arc. This accuracy is about the same as that which is achievable using inductive-type rotary encoders. Therefore, the high degree of tolerance to misalignment achievable with inductive-type rotary encoders results in reduced manufacturing tolerances and significant cost savings.
Another advantage of inductive-type rotary encoders is that the phase of the rotor output signal varies almost linearly from 0-2.pi. as the rotor rotates one line pair. This enables multiplication of the basic output counts (to be described below) per revolution by as much as 60 times. This can yield an encoder giving 14,000 counts per revolution.
The use of inductive-type rotary encoders is well known. See, for example, U.S. Pat. Nos. 3,247,504 to Emmerich; 3,812,481 to Stednitz; and 4,358,723 to Scholl et al, the disclosures of which are herein incorporated by reference.
U.S. Pat. No. 4,358,723 to Scholl et al discloses a method and apparatus for measuring rotation using a resolver which outputs a standard train of pulses and also includes a once-per-revolution optical indicator. See column 2, lines 47-51, column 4, lines 36-46, and column 7, lines 8-28. A basic rotor and stator layout is disclosed in FIG. 2, elements 14' and 14". The output from the once-per-revolution optical indicator is used to compensate for manufacturing inaccuracies in the resolver.
U.S. Pat. No. 3,812,481 to Stednitz discloses a non-contacting electrical rotary position and rotation transducer which utilizes inductive coupling. The rotor includes at least one undulating or crenelated winding which is short-circuited. The rotor winding is energized by an energizing winding placed on a stator, by inductive coupling. The stator has at least one additional winding (sensing winding) thereon which matches the outline and configuration of the short-circuited rotor winding. Current flow is induced in the one or more stator sensing winding(s) by the rotor short-circuited winding(s). Thus, no contacts need to be provided for the winding(s) located on the rotor. Winding configurations for inducing one pulse per revolution when the rotor and sensing windings are in congruence are disclosed in FIGS. 3 and 4.
U.S. Pat. No. 3,247,504 to Emmerich discloses a digital resolver system which uses a stator and rotor pattern. Shaft position is determined by means of a digital resolver system.
Farrand Controls, a division of Farrand Industries, has marketed a precision linear and rotary position transducer under the tradename of Inductosyn.